1. Field Of The Invention
The present invention relates to a powertrain component for use in an internal combustion engine. More particularly, the invention relates to a component having a hard, wear resistant amorphous or nanocrystalline ceramic coating system of constant, abruptly varying or continuously varying composition deposited thereon.
2. Related Art Statement
The selection of materials from which internal combustion engines and associated machinery are fabricated is subject to constraints which grow more stringent with demands for lower weight, increased efficiency, reduced internal friction and reduced emissions. These factors are strongly synergistic. For example, reduction in the mass of a moving component reduces total vehicle weight, allows higher engine speeds and increased specific power output while simultaneously reducing the forces and therefore the friction associated with guiding its motion, so reducing vibration and stress on other components. This in turn allows further weight reductions elsewhere. Also, materials with sufficiently low friction and wear in dry sliding need not be lubricated at all, thus eliminating the parasitic power loss required for pumping oil, further increasing engine efficiency.
Development of new powertrain materials requires simultaneous control of bulk properties such as weight, strength and fatigue resistance, and surface properties, such as friction, wear resistance, chemical stability and lubricant compatibility.
New, lighter-weight powertrain materials fall into two general categories: (1) light-weight metals such as titanium, magnesium, aluminum, and titanium-, magnesium- and aluminum-based alloys; and (2) ceramics such as silicon-carbide and silicon-nitride. While all of these are strong, light, and fatigue resistant, each suffers from one or more failings in the powertrain application. For example, the light metals and their alloys tend to exhibit poor wear resistance and may fail catastrophically in an oil starved operating condition. In turn, ceramics cannot be cast or easily machined to net shape, and so are difficult to form to their final shape with high accuracy at low cost. Furthermore, some ceramics may not be compatible with current lubricant formulations and are subject to rapid wear in sliding contact.
Illustrative is EP 435 312 Al (published Jul. 3, 1991) which discloses a hard and lubricous thin film of amorphous carbon-hydrogen-silicon and a process for producing the film, which involves heating the component (hereafter sometimes referred to as the "substrate") to 600.degree. C. in a vacuum. The disclosed film was applied to an iron-based (ferrous) material, resulting in a hard coating with low friction. However, such temperatures are incompatible with most substrates of interest, which lose desirable properties, soften, or even melt at such temperatures. Another approach, disclosed in U.S. Pat. No. 4,909,198 which issued on Mar. 20, 1990 has been to spray a thick (100-200 microns) iron or steel film which imparts the friction properties of conventional iron engine materials to an aluminum alloy component. That method may result in an engineered surface equivalent to that of current iron and steel materials, but is intrinsically incapable of providing a superior surface.